/Engine/Plugins/Online/OnlineServices/Source/OnlineServicesInterface/Public/Online/Auth.h
Some auth interfaces have more than one method for providing credentials when linking to an external account. An example usage is when the auth interface can provide a token for linking the local hardware device as the primary method before falling back to a token linked to the users online account.
namespace UE { namespace Online { enum EExternalAuthTokenMethod { Primary, Secondary, } } }